The art is replete with successive generations of magnetic tape drives. In general, the heart of a magnetic tape drive is comprised of a magnetic head for reading (and/or writing) the magnetic tape that passes over it. As is known in the art, when the magnetic tape passes over a magnetic head, the magnetic signals impressed in the tape are read by the magnetic head and sent on to other electronic systems for processing and amplification. On the other hand, a magnetic head may also be used to write or send data for storage onto a magnetic tape passing over it.
In a tape drive, the magnetic tape is normally fed out from one reel, across the magnetic head, and collected by a second reel. The tape path from one reel to the other typically does not form a straight line, and typically forms an arc across the magnetic head. See, for example, FIG. 1.
The head in FIG. 1 is supported by a stepper motor which in turn is attached to the frame of the magnetic recording device. As described in more detail below, the stepper motor provides for a coarse adjustment of the head in a direction transverse to the tape path (shown by the arrow in FIG. 1). Interposed between the stepper motor and the head (but not shown in FIG. 1) are elements that provide for a tracking adjustment of the head, which are described further below with respect to FIG. 3.
Also shown in FIG. 1 are guides on each side of the head which constrain the transverse motion of the tape as it passes over the head. The guides in FIG. 1 are shown to be rollers with rotating flanges. Prior guides have also used, for example, rollers with non-rotating flanges or spring fingers.
Prior art tapes have multiple tracks, in order to maximize the storage capacity of the tape. FIG. 2a, for example, shows a tape head having read, write and read segments. The three segments allow the tape to read while being written in either direction, in order to confirm the accuracy of the written data. Each segment of the head in FIG. 2a is shown to have a series of "gaps" aligned in the direction of movement of the tape, where data is either read from or written to the tape passing over it, thus creating or reading a separate track. Since the normal movement of the tape is left to right in FIG. 2a (or vice versa), the tracks form a line along the length of the tape.
The bottom (fifth) track in FIG. 2a is shown to be a pre-written servo track used to position the tape. As described below, the signal read from the servo track is used to align the transverse position of the head and the tape. A typical servo "track" is actually comprised of two tracks, each of which have different frequencies. The head read element runs in between them, thus reading both signals and processing them to keep the gap centered between the two servo tracks. Thus, the servo track in FIG. 2a is actually comprised of two tracks, labeled X and Y. By keeping the fifth read gap aligned between servo tracks X and Y, the tape is maintained in a transversely fixed position with respect to the head. As a result, the magnetic data tracks that are written by the other write gaps (tracks 1-4) are at a fixed transverse distance from the servo tracks. When read, the servo track maintains each track properly aligned with that track's respective read gap on the head.
As shown in FIG. 2b, by transversely repositioning the alignment of the tape and the head, another series of data tracks can be written to (and subsequently read from) the tape. Thus, the data tracks in FIG. 2b having the "a" suffix can be envisioned as corresponding to the tracks in FIG. 2a and created by tracking the tape using servo tracks X and Y. By transversely repositioning the head and the tape so that the fifth gap of the head lies between servo tracks Y and Z, a second series (labeled with the suffix "b") can be created (and then read) at each gap position. As seen, the head and the tape must be moved transversely with respect to each other by at least one gap width (or, equivalently, one track width), so that the adjacent tracks are sufficiently separated in the transverse direction.
A series of data tracks can be created (and read) in this manner. Thus, FIG. 2b could also have a series of tracks with a suffix "c". A series of three tracks would require a fourth servo track located below servo track Z. The number of tracks that can be created in this manner depends on the width of the gaps and the space between them.
In general, the initial positioning of the head with respect to the tracks that are to be read is done by a stepper motor that, as shown in FIG. 1, supports the head and is affixed to a frame of the recording device. Thus, if the series of tracks with the suffix a are to be read in FIG. 2b, the stepper motor positions the lower read gaps of the head nominally between servo tracks X and Y. If the tracks having suffix b are to be read, the stepper motor moves the nominal position of the head between servo tracks Y and Z.
During recording or playback, the tape must also be "tracked" in order to maintain the gap on the head between servo tracks X and Y (or Y and Z) once initially positioned there. The tape guides or the tension in the tape only provides a coarse transverse constraint of the transverse movement of the tape. Also, a small air film may be present between the head and the tape. Thus, the tape is found to "wander" in the transverse direction as it passes over the head.
In one known way of tracking the tape, or for positioning the tape with respect to a particular servo track, the head is movable with respect to the tape. FIG. 3 represents the elements of a prior art method for tracking the tape. (In FIG. 3, the length of the tape runs in and out of the page; thus, FIG. 3 is a view along the direction of motion of the tape shown in FIGS. 2a and 2b, toward the head.)
The head 10 is supported by leaf springs 30a, 30b, which allow movement of the head in the transverse position. The ends of leaf springs 30a, 30b opposite the head 10 are attached to support 40. Support 40 is attached to frame 20 of the magnetic recording device via stepper motor 25. (In order to focus on the tracking elements interfacing with the head, the stepper motor has been reduced in representation in FIG. 3, but has been maintained in the figure to show spatial relationships.) Thus, after the stepper motor 25 positions the head 10 to read a particular series of tracks, as described above, the support 40 remains stationary with respect to the frame 20 of the magnetic recording device.
A magnet and coil device 50, analogous to a voice coil in a speaker, serves to move the head 10 transversely. The magnet portion 54 is rigidly attached to the head 10 and moves transversely with the head 10. The magnet portion 54 is surrounded by coil portion 52, which is attached via the support 40 to frame 20. The magnet and coil portions 52, 54 are positioned so that the magnet portion 52 can move in the transverse direction in response to a change in current in the coil 52.
The motion of the magnet portion 52 serves to also move the head 10 in the transverse direction. (The direction of motion in the transverse direction is determined by the direction of the current through the coil portion 52.)
The transverse wander of the tape described above is fairly continuous. The wander creates a change in the signals from the servo tracks that are read by the magnetic head. The processing of the change in the servo signal maintains an appropriate driving current at the coil so that the head moves transversely by a corresponding amount and thus "follows" the tape wander. In this manner of tracking, the head continues to follow the tape wander in a substantially continuous manner.
The change in current in the coil portion 52, which serves to move the head 10 transversely, is generated by electronic processing of the tracking signal received by reading the servo track, as shown for example in FIG. 2a or 2b. For example, if the read servo tracking signal indicates that the head has moved with respect to the tape transversely either up or down, the processing electronics will generate a change in current in the coil portion that moves the head 10 in the proper direction until the servo track is properly positioned over the gap.
Formats of tracking signals and the electronics used to read those signals and process an appropriate signal for the coil portion in order to adjust the transverse position of the head are well known in the art. Alternative tracking methods, such as optically detecting the edge of the tape, are also known.
The prior art practice of moving the head transversely with respect to the tape presents a number of difficulties. For example, although the head shown in FIG. 3 might be supported by movable leaf springs, the head also has relatively heavy electrical wires that extend to write and read amplifiers that are fixed to the frame. These wires interfere with the smooth movement and precise positioning of the head in the transverse direction by the voice coil 50 mechanism. Such imprecision is especially unsuited for future generations of tapes, which will have narrower tracks than ever before.
Also, the fixed tape guide and moving head is not well suited for the future generations of tapes. Future generations of tapes will not only be thinner and smoother, but will also have to run at higher speeds and demand improved wear characteristics (for example, an increase in the number of passes before the tape edge is degraded). Thinner tape requires lower tension in the tape drive. The lower tension, however, creates a haphazard packing of the tape on the reel.
That is, once the tape clears the guide (see FIG. 1) on the way to the reel, single winds of the tape on the reel are not well aligned, but become transversely shifted with respect to each other. (See FIG. 3a, a cross-section of a tape loosely wound onto a reel.) The transverse misalignment is the result of the reduced tape tension along with the air entrapped between the tape on the reel and the portion of tape being wound onto the reel.
Consequently, when the tape is wound off of the reel, the lateral force that may be imposed on the edges by the guides before passing over the head can be considerable. This can lead to rapid degradation of thin tapes.
An example of a prior art device in a manner similar to that described above with respect to FIGS. 1-3 is shown in U.S. Pat. No. 5,414,578. It is also known in the prior art to move the entire tape guide for tracking. The guide, however, is also relatively heavy. Thus, tracking the tape by moving the entire guide is not suited the precise tracking of thin, low tension tapes having a high density of tracks.